The present invention relates in general to high rate digital transmissions over existing telephone lines including local subscriber loops and more particularly to a method and system of connecting telephones and high rate modems directly to the same telephone line without a splitter.
The increased use of telephone twisted pair wiring for data communications has resulted in a push for faster modems and improved signaling protocols compatible with the public switch telephone network (PSTN). An example includes the emerging modem communication standards which have pushed the limit of transmission speeds close to 56 Kbps. Another example includes the emerging variety of the DSL communications protocols including asymmetric digital subscriber line (ADSL), symmetric digital subscriber line (SDSL), high bit rate digital subscriber line (HDSL), and very high rate digital subscriber line (VDSL). Each DSL variant represents a different transmission speed over a different distance of copper pair wiring. Currently, industry is producing equipment capable of implementing one or more of the DSL protocols. Modems, routers, line cards, and digital loop carrier systems are all examples of such equipment.
In principle, a DSL modem and a plain old telephone system (POTS) or other voice band device can operate simultaneously over the same wire line pair since they use different frequency bands. The connection, however, of a POTS to the same wire line pair as a DSL modem can suffer from several problems that result from the change in input impedance of the POTS as it is added to the line. In essence, a POTS is not designed to handle frequencies outside the voice band of 300-3.4 kHz. In addition, a POTS often contains nonlinear components that may create harmonic interferences. Examples of such nonlinear components include Zener diodes, transistors, varistors, triacs, and other devices used for overvoltage protection, sidetone generation and overvolume protection. For specific details on the circuitry of the POTS, reference is made to Whitman D. Reeve, xe2x80x9cSubscriber Loop Signaling and Transmission Handbook: Analog,xe2x80x9d IEEE Press, 1992, included herein by reference.
In a splitterless configuration, when the POTS goes off-hook during a DSL transmission, these components can distort the DSL signal causing intermodulation and producing nonlinear echos of the upstream DSL signal. Harmonics of these echos lie in the downstream DSL band, and thus severely corrupt the downstream signal. To overcome these problems, a POTS splitter may be used to isolate the DSL high frequency signals from the voice band frequencies.
In principle, a splitter is a combination of low-pass and high-pass filters. The low-pass filter is used to isolate the voice band signals and route them to/from the telephone and the central office. The high-pass filter is used to isolate the passband DSL signal and route it to and from the DSL modem and the central office. The splitter can be part of the DSL modem, in which case the telephone(s) has to be connected to the modem. Alternatively, the splitter can be installed at the line drop to the home, which may require professional installation and re-wiring of the telephone lines going to the DSL modem inside the customer premises.
Besides the installation issues which the customer may be faced with, the use of a splitter in a DSL modem adds to the overall complexity and cost of the user""s communication equipment. The filters, transformers, resistors and other discrete components needed to implement a particular splitter design can be bulky and expensive. In addition, the application programs and the control logic necessary to operate the splitter and cause it to activate voice band or DSL filtering functions also increase overall complexity of the system. Utilizing separate connections for the voice band and DSL functions would allow the subscriber to utilize separate devices, although such a configuration restricts the way a customer can connect the equipment and adds the costs of a separate line. It would be advantageous to utilize a DSL modem in the same manner in which voice band modems are currently operated.
The present invention is a solution for connecting a POTS and DSL modem directly to the same line without a splitter, thus enabling the customer to plug the DSL modem and a normal telephone in any phone jack in the home similar to the way voice band modems are installed.
According to one embodiment, the invention discloses a communications device that accommodates changes in transmission line characteristics over a wire line pair that connects a customers plain old telephone system (POTS) to the central office or other centralized call exchange facility. The device includes a transceiver with an analog interface coupled to the wire line pair and arranged to communicate with the central office. An on/off-hook detector is coupled to the analog interface and arranged to detect when the POTS seizes the line corresponding to the off-hook state of the POTS. The device includes an impedance analyzer function coupled to the on/off-hook detector and the analog interface with an output so that the difference of line impedance between the on-hook and off-hook conditions of the POTS can be determined. The output of the impedance analyzer function is fed to a control logic function that is configured to adjust the transmission parameters of the transceiver in response to changes in the transmission characteristics of the line pair.
The communications device is able to compute and store transmission line profiles corresponding to the on-hook and off-hook states of the POTS. Since the impedance of the customer premise equipment changes as the POTS goes from on-hook to off-hook, the transmission line profiles contain transmission line parameters which the communications device can use to avoid the effects of nonlinearities produced as the POTS transitions between states. The transmission line parameters can include the transmission power levels, echo cancellation coefficients, and equalizer coefficients used by the transceiver during a DSL connection.
In another embodiment, disclosed is a digital subscriber line (DSL) modem capable of operating with multiple transmission line profiles depending on the current line characteristics of a wire line pair coupling a customer to the central office. The DSL modem includes an interface to the Wire line pair and a signal converter with an output terminal coupled to the interface. An on/off-hook detector drives an impedance analyzer function that is able to measure and produce a figure of the impedance presented to the wire line pair by the combination of the DSL modem and the voice band equipment. The impedance analyzer function is coupled to control logic which, in turn, is configured to determine the transmission line characteristics of the wire line pair as the voice band device transitions between states.
In one embodiment, the DSL modem includes a memory space, such as Random Access Memory (RAM), EEPROM, Flash memory, magnetic hard disk, or other suitable memory configuration. The memory space is accessible by the control logic block and stores transmission line profiles computed by the impedance analyzer function and the control logic block. The profiles can contain transmission line parameters corresponding to the on-hook and off-hook conditions of the voice band device coupled to the wire line pair. Examples of such transmission line parameters include the upstream power, echo cancellation and equalizer coefficients used by the DSL modem transceiver during on-hook and off-hook operation of the voice band device. In one embodiment the DSL modem is able to dynamically switch from one transmission line profile to another as it detects transitions of the voice band equipment between states.
Also disclosed is a method of simultaneously operating a digital subscriber line modem and a voice band device, such as a telephone or voice band modem, over the same connection. The method includes detecting when the voice band modem is on-hook and creating an on-hook transmission profile for the DSL modem which is used to maintain a DSL session over the connection when the voice band device is on-hook. The method could also include the steps of detecting when the voice band device is off-hook and creating an off-hook transmission profile for the DSL modem that is used to maintain a DSL session over the connection when the voice band device is off-hook.
The on-hook and off-hook transmission profiles can be stored for future use during the on-hook and off-hook conditions of the voice band device, respectively. The transmission profiles include a set of transmitter and receiver co-efficients that permit the DSL modem to operate within a substantially linear operating region directly coupled voice band telephone equipment. The coefficients can include the equalization and echo cancellation coefficients used by the transmitter and receiver sets of a DSL modem. In creating the off-hook transmission profile, the power of the DSL modem can first be reduced resulting in less intermodulation distribution by the voice band smaller harmonics being generated near the voice band spectrum and the DSL receiver spectrum. The on-hook and off-hook transmission profiles can be recalled by the DSL modem depending on the current state of the voice band device. The recall of transmission profiles can occur dynamically as the voice band device transitions from on-hook to off-hook. This provides near-uninterrupted use of both the voice band device and the DSL modem over the same connection.
Also disclosed is a method of training a DSL modem to operate simultaneously over the same connection as a voice band device. The method includes the steps of placing the voice band device in an on-hook state and establishing a DSL connection followed by explicit storage of the on-hook transmission profile. The training signals are analyzed in order to determine a set of on-hook transmission coefficients that allow simultaneous operation of the DSL modem with the voice band device in the on-hook state. The on-hook transmission line coefficients can be saved for future use as an on-hook profile.
The method can also include the steps of placing the voice band device in an off-hook state while transmitting a single pilot tone in the upstream direction that is analyzed in order to determine the linear and nonlinear operating ranges of the voice band device and the levels at transmit attenuation that must be applied in the DSL modem. Next, a set of off-hook transmission line coefficients that allow simultaneous operation of the DSL modem with the voice band device in the off-hook state are computed. The transmit attenuation and the coefficients permit the DSL modem to avoid the nonlinear amplitude regions of the voice band device in its off-hook state. The off-hook transmission line coefficients can be stored for future use in an off-hook profile. In the most severe requirements for upstream power cutback, the use of a single pilot tone within an upstream communication channel provides a low rate upstream control channel.
An alternative method of training a DSL modem to operate over the same connection as a voice band device is disclosed. With the alternative method the voice band device is placed in an off-hook state and upstream power is reduced in order to increase the robustness of the transmission channel. In one embodiment, 4-Quadrature Amplitude Modulation (4-QAM) is used as the subchannel modulation protocol. The upstream power and subchannel modulation channels can be stored for future use by the DSL modem. In another embodiment, the upstream power level of the DSL modem is set to a predefined reduced level and never scaled either up or down. Alternatively, upstream transmissions can be reduced to a single pilot tone that could be modulated to provide a minimal very low rate upstream control channel.
An advantage of the invention is that it allows full asymmetric digital subscriber line performance when the voice band equipment is in the on-hook state. In the off-hook state the system adapts parameters and allows the throughput to be reduced in order to limit interference effects caused by nonlinearities in the voice band.
Another advantage of the invention is that it permits a DSL modem design without a splitter and thus reduces the cost and complexity of customer premise equipment.
Still another advantage of the invention is that it permits concurrent and simultaneous use of the same wire line pair for both voice band and DSL communications.